In oil well pumps, the walking beam at the well head is connected by a sucker rod to the pump at the bottom of the well. The sucker rod is usually of great length, often in excess of 10,000 feet and must be of high tensile strength to support even its own weight. Furthermore, the rod is immersed in the crude oil of the well which is highly corrosive. The rod for a well is made up of many individual rod-lengths which are joined end-to-end, each rod-length being about 25 feet long. The coupling used to join the rod-lengths typically uses a threaded fitting.
One of the difficulties encountered in long strings of sucker rod is the weight of the rod itself. In long strings, it is a common practice to use rods of successively smaller diameter from top to bottom of the string to provide the necessary tensile strength and minimize the weight of the rod at the bottom of the string. The weight of the string itself causes the individual rod-lengths to be elongated. When the rod is reciprocated during pumping the alternate loading and unloading of the rod causes cyclical stretching and, in some cases there is considerable disruption or interference with the stroking of the pump. In addition to the extremely high tensile stress and the cyclical loading, the rod is subjected to bending stress due to changes in the direction of the well casing. Thus, breakage of the sucker rod is not uncommon and it causes significant amounts of down time of the pumping apparatus while the rod is removed from the weld shaft and replaced.
One widely used structure of a rod-length is commonly known as a "three-piece rod". In this structure, the rod is threaded at both ends and a rod-end fitting is threadedly engaged with each end of the rod to form a three-piece assembly. The free end of the rod-end fitting is adapted for connection to another rod-end fitting. One form of three-piece rod assembly uses, on each end of the rod, a fitting which has a threaded male pin on the free end, such fitting being known as the "pin-end". With this, the three-piece rods are joined to each other by a conventional threaded coupling which is screwed onto adjoining pin-ends. In another form of three-piece rods, one end of the rod is fitted with a pin-end and the other end has a fitting with a threaded socket at its free end, the latter being called a "box-end". With this, the rods are joined together by screwing the pin-end of one into the box-end of the other.
In the prior art, sucker rods for use in oil wells are commonly made of steel, largely because of its high tensile strength. However, it does have the drawbacks of weight and susceptibility to corrosion. Attempts have been made to use fiber glass rods with the attendant advantages of lightweight and corrosion resistance. Also, it has been proposed to use aluminum rods to gain the advantages of lightweight, corrosion resistance and potential cost effectiveness. However, there is a need for a satisfactory coupling between the rod-lengths.
A general object of this invention is to provide an improved sucker rod assembly that overcomes certain disadvantages of the prior art.